Conventional mixers for mixing chemicals (typically in gaseous form) in slurries, such as the MC.RTM. mixer sold by Kamyr, Inc. of Glens Falls, N.Y. and the "Ahlmixer" TM sold by Ahlstrom Machinery of Roswell, Ga. are utilized for mixing chlorine, chlorine dioxide, oxygen, and like chemicals, in liquid and/or gaseous form, into paper pulp slurries having consistencies ranging anywhere from about 1 to 16%. These mixers typically have a housing with a main hollow body portion with a generally circular cross-section and a rotatable impeller disposed in the hollow body portion for imparting a circular and tangential force to the slurry within it, as part of the mixing action for intimately mixing the slurry (pulp) and chemical. The mixed slurry is then discharged through a radial discharge outlet in the main body portion of the housing. These mixers are very successful in accomplishing their desired end results.
In prior art mixers for mixing chemicals in slurries, particularly in the pulp and paper field, the slurry often has substantial amounts of gas within it. Of course the gas content is significantly increased during mixing if the chemical being mixed with the slurry is in gaseous form. Therefore, under some circumstances there is separation of gas at the discharge outlet due to the abrupt transition from circular and tangential flow in the main housing portion to radial flow in the radial discharge outlet. Of course the gas separation, if it occurs, tends to reduce mixing efficiency, and can also serve as an impediment to uniform flow of the mixed slurry out of the discharge outlet. Under these conditions, a gas bubble typically forms adjacent the leading wall (in the direction of circular movement of slurry within the main body portion of the housing) which extends a significant distance into the discharge outlet.
According to the present invention, methods of mixing a chemical with the slurry are provided which effect discharge of the mixed chemical and slurry from the main body portion of the housing without significant separation of gas from the slurry under any conditions, and a mixer is provided which achieves these results.
According to one aspect of the present invention, a method of mixing chemical with slurry, having gas present in the slurry (which may be inherent in the slurry, and/or may be as a result of the gaseous form of the introduced chemical), using a mixer housing having a main hollow body portion with a generally circular cross-section and radial discharge for mixed slurry/chemical, is provided. The method comprises the following steps: (a) Introducing the slurry and chemical separately into the mixer. (b) Acting upon the slurry and chemical in the mixer to mechanically intimately intermix them, including by moving the slurry and chemical together in a circular and tangential path in the main body portion of the mixer. And, (c) discharging the mixed chemical and slurry from the main body portion through the radial discharge without significant separation of gas from the slurry by minimizing the transition of the mixed chemical and slurry from its circular and tangential path in the body portion to a radial path in the radial discharge.
Step (c) is typically practiced by shaping the leading (in the direction of circular movement of slurry within the body portion) wall of the radial discharge so that it presents a curved configuration to the slurry entering the discharge. The curvature of the curved configuration cannot exceed an angle of about 10 degrees at any point along it until radial flow is established. This thus provides a smooth transition from circular/tangential flow to radial flow. This smooth transition may be further enhanced by providing a curved configuration at the trailing wall too which substantially parallels the contour of the leading wall, again having a curvature which does not exceed an angle of about 10 degrees at any point along it, until radial flow is established. The shaping of the walls may be accomplished by providing inserts, which inserts approximate the configuration of a gas bubble that would normally form in the radial outlet, and a dead space opposite the gas bubble.
The invention also comprises a mixer which has the features recited above. That is the mixer includes as a distinguishing component from the prior art, means associated with the radial discharge for minimizing the transition of the mixed chemical and slurry from its circular and tangential path in the main body portion to a radial path in the radial discharge, so that no substantial separation of gas from slurry in the discharge takes place. The transition minimizing means preferably comprises means defining the shape of the leading wall of the radial discharge so that it presents a curved configuration to the slurry entering the discharge, in which the curvature of the configuration does not exceed an angle of about 10 degrees at any point along it until the radial flow is established. The transition means also comprises a parallel curvature of the trailing wall of the radial discharge. These configurations may be provided by inserts which preferably are of a material compatible with the discharge outlet, and consistent with process conditions (e.g. stainless steel, carbon steel, titanium, Hastelloy, etc.). The configuration of the outlet may be circular, quadrate, or a transition from quadrate to circular.
According to another aspect of the present invention, prevention of significant gas separation at the discharge from a mixer having a radial outlet is accomplished in another way. According to this aspect of the invention, the existing radial discharge outlet is removed and retrofit with one that it is substantially tangential, similar to a pump volute, with a gradually increasing cross-section By tangentially discharging the mixed chemical and slurry from the body portion of the mixer through the discharge, again the slurry does not change direction precipitously, so that substantial separation of gas from the slurry at the discharge is avoided, even if the chemical mixed with the slurry is introduced into the mixer in gaseous form.
It is the primary object of the present invention to minimize gas separation at the discharge outlet of a mixer for mixing slurry and chemical. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.